diff --git a/simgear/scene/sky/cloud.cxx b/simgear/scene/sky/cloud.cxx
index d4557064..dbc7d03d 100644
--- a/simgear/scene/sky/cloud.cxx
+++ b/simgear/scene/sky/cloud.cxx
@@ -46,11 +46,14 @@ inline int (isnan)(double r) { return isnan(r); }
 #endif
 
 #if defined (__FreeBSD__)
-inline int isnan(double r) { return !(r <= 0 || r >= 0); }
+#  if __FreeBSD_version < 500000
+     extern "C" {
+       inline int isnan(double r) { return !(r <= 0 || r >= 0); }
+     }
+#  endif
 #endif
 
 
-
 static ssgStateSelector *layer_states[SGCloudLayer::SG_MAX_CLOUD_COVERAGES];
 static bool state_initialized = false;
 static bool bump_mapping = false;
diff --git a/simgear/sound/soundmgr_openal.cxx b/simgear/sound/soundmgr_openal.cxx
index e9e2dfbc..9ec8355d 100644
--- a/simgear/sound/soundmgr_openal.cxx
+++ b/simgear/sound/soundmgr_openal.cxx
@@ -46,6 +46,15 @@ inline int (isnan)(double r) { return isnan(r); }
 #define isnan(x) _isnan(x)
 #endif
 
+#if defined (__FreeBSD__)
+#  if __FreeBSD_version < 500000
+     extern "C" {
+       inline int isnan(double r) { return !(r <= 0 || r >= 0); }
+     }
+#  endif
+#endif
+
+
 #if defined (__FreeBSD__)
 inline int isnan(double r) { return !(r <= 0 || r >= 0); }
 #endif